The present invention relates generally to single extruder systems for processing, screening and extruding foamable plastic material. More particularly, the invention relates to a system for flowing plastic material through a screen changer at an upstream point where the material is at relatively high temperature and pressure.
To manufacture foam plastic products, it is known to flow molten foamable material through a screw extruder, then through a screen changer, and then through a die. The molten material is cooled as it flows through the extruder. The cooled material should have sufficient viscosity to prevent premature foaming.
High viscosity material, however, may require a larger or more complicated screen changer. If the screen changer is not large enough, the pressure drop across the screen changer may cause premature foaming. Also, the increased pressure demand at the end of the extruder results in more shear heat in the extrudate, lessening cooling efficiency.
A prior art screen changer is shown in U.S. Pat. No. 3,856,277 (Tiramani).